Javascript array reduce return zero instead of undefined

Question

I've got an example array that I'm trying to reduce by the counts of the occurrence of a key (sentiment in this example):

const sentimentLog = [
  {
    id: 1,
    createdOn: new Date('2020-02-13'),
    sentiment: 1
  },
  {
    id: 2,
    createdOn: new Date('2020-02-12'),
    sentiment: 1
  },
  {
    id: 3,
    createdOn: new Date('2020-02-12'),
    sentiment: 2
  },
  {
    id: 4,
    createdOn: new Date('2020-02-11'),
    sentiment: 3
  },
  {
    id: 5,
    createdOn: new Date('2020-02-11'),
    sentiment: 2
  },
  {
    id: 6,
    createdOn: new Date('2020-02-10'),
    sentiment: 1
  },
  {
    id: 7,
    createdOn: new Date('2020-02-10'),
    sentiment: 2
  },
  {
    id: 8,
    createdOn: new Date('2020-02-09'),
    sentiment: 1
  }
]

I'm using:

const sentimentGrouped = (sentiments) => {
  return sentiments.reduce((hash, { sentiment }) => {
    hash[sentiment] = (hash[sentiment] || 0) + 1
    return hash
  }, [])
}

And it's nearly there. What I can't figure out is how to replace undefined when there's no sentiment scores of 0 (which is a possibility).

console.log('sentimentGrouped', sentimentGrouped(sentimentLog))

The above produces:

"sentimentGrouped" [undefined, 4, 3, 1]

Whereas I'd like:

"sentimentGrouped" [0, 4, 3, 1]

What am I missing?

Thanks in advance.

Edit: I'll elaborate a bit further, there's 4 scores that will be returned (0 to 3). The data returned will be based on a date range. So there may be instances where there'll be no 1s returned, similarly no 3s returned by a different date range.

Is there a maximum sentiment value? Wondering if you can prefill the array with all zeros. — Nicholas Tower
– Nicholas Tower, Commented Feb 14, 2020 at 15:03
It's between 0 and 3 - so 4 options only - the UI represents these as smiley faces :) — Neil Merton
– Neil Merton, Commented Feb 14, 2020 at 15:04
beside pre populate the array with zeros (as you know the length of it in advance) is there a problem using an object as the hash? — Sagiv b.g
– Sagiv b.g, Commented Feb 14, 2020 at 15:08
I want to avoid an object, as I'm using the result within a v-for loop in Vue.js — Neil Merton
– Neil Merton, Commented Feb 14, 2020 at 15:09

Nicholas Tower · Accepted Answer · 2020-02-14 15:08:12Z

4

The issue is that if you never touch an element of the array, then it stays as a hole in the array, which means it's treated as undefined. Since you know the length of the array i would just prefill the array with zeros. Any sentiment score that does occur will be incremented. Any one that doesn't will stay with its initial value.

return sentiments.reduce((hash, { sentiment }) => {
  hash[sentiment] = hash[sentiment] + 1
  return hash
}, [0, 0, 0, 0])

edited Feb 14, 2020 at 15:08

answered Feb 14, 2020 at 15:06

Nicholas Tower

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